Silicon, germanium, zirconium, and titanium metal halides are often used as vaporous reactants in forming metal oxide glasses. For example, hydrolysis of SiCl.sub.4 has been the industry preference for producing high purity silica over the years. The oxidation of SiCl.sub.4, through pyrolysis and hydrolysis, however, has the disadvantage of producing chlorine or a very strong acid by-product, hydrochloric acid (HCl). Hydrochloric acid is detrimental not only to many deposition substrates and to reaction equipment but also is harmful to the environment. Emission abatement systems have proven to be very expensive due to loss and maintenance of equipment caused by the corrosiveness of HCl.
As an alternative, high purity quartz or silica has also been produced by thermal decomposition and oxidation of silanes. However, this requires taking safety measures in handling because of the violent reaction that results from the introduction of air into a closed container of silanes. Silanes react with carbon dioxide, nitrous oxide, oxygen, or water to produce high purity materials that are potentially useful in producing, among other things, semiconductor devices. However, silanes have proven to be much too expensive and reactive to be considered for commercial use except possibly for small scale applications requiring extremely high purity.
U.S. Pat. No. 5,043,002 to Dobbins et. al, which is relied upon and incorporated by reference, proposed alternative silica precursor materials. This patent disclosed bubbling a carrier gas through a silicon-containing reactant compound, preferably a halide-free compound such as polymethylsiloxanes, in particular, polymethylcyclosiloxanes such as hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane ("OMCTS"), and decamethylcyclopentasiloxane. A mixture of the reactant compound vapor and nitrogen is transported to the burner at the reaction site, where the reactant is combined with a gaseous fuel/oxygen mixture and combusted. U.S. Pat. No. 5,152,819 to Blackwell et al., the disclosure of which is relied upon and incorporated by reference, describes additional halide-free silicon compounds, in particular, organosilicon-nitrogen compounds having a basic Si-N-Si structure, siloxasilazones having a basic Si-N-Si-O-Si structure, and mixtures thereof, which may be used to produce high purity silica glass without the concomitant generation of corrosive, polluting by-products.
Although use of halide-free silicon compounds as feedstocks for silica glass production avoids the formation of HCl, some problems remain, particularly when the glass is intended for the formation of high purity bulk fused silica and high quality optical products such as optical waveguides. For example, as disclosed in copending U.S. patent application No. 08/574,961 entitled "Method for Purifying Polyalkylsiloxanes and the Resulting Products," which is relied upon and incorporated by reference, the presence of high boiling point impurities in, for example, a polyalkylsiloxane feedstock, can result in the formation of gel deposits in the vaporization and delivery systems carrying the vaporous reactants to the burner or within the burner itself. Such polymerizing and gelling of the siloxane feedstock inhibits the controllability and consistency of the silica manufacturing process. This problem is more prevalent when an oxidizing carrier gas such as oxygen is included in the reactant vapor stream, because oxidizers appear to catalyze polymerization of the siloxane feedstock. Such polymerizing and gelling reduces the deposition laydown rate of the bulk silica soot or soot preform that may be subsequently consolidated to a blank from which an optical waveguide is fabricated.
An additional problem encountered when silica soot or silica preforms are formed using siloxane feedstocks is that particulates of the high molecular weight, high boiling impurities may be deposited in the bulk silica soot or on the optical waveguide fiber blank, resulting in "defect" or "clustered defect." Defects or clustered defects are imperfections that adversely affect the optical and structural quality of the optical waveguides formed using the silica soot.
In copending application Ser. No. 08/767,653, the content of which is relied upon and incorporated by reference, it is disclosed that the clustered defects could be reduced by delivering a liquid siloxane feedstock to a conversion site, atomizing the feedstock at the conversion site, and converting the atomized feedstock at the conversion site into silica. One way to atomize the feedstock at the conversion site involves pneumatically or "airblast" atomizing the liquid siloxane feedstock by delivering the liquid feedstock to the conversion site with an inert gas.
Although atomizing the liquid siloxane feedstock proximate the conversion site reduces clustered defects, such a liquid delivery system presents several further challenges. For example, increasing the atomizing gas velocity desirably produces smaller liquid droplets, which are more readily vaporized and burned in the burner flame. Smaller droplets are desirable because larger droplets cause wart-like defects ("warts") on the surface of the soot blank. In addition, smaller droplets can be more easily focused with the surrounding gases to produce a more focused deposition stream. On the other hand, increasing atomizing gas velocity adds turbulence to the burner flame, which reduces the soot capture rate and is one cause of a physical soot defect known as "lizard skin." Lizard skin is a term for a rough soot blank surface.
Accordingly, it would be desirable to provide a method in which a liquid delivery system could produce a focused deposition stream containing small droplets without a high gas velocity, and in which there is low burner flame turbulence.